The present invention relates generally to liquid crystal devices used as light modulators for the projection of images. More particularly, the invention relates to a liquid crystal cell upon which information can be written by a laser beam for subsequent projection. The cell operates in a transmissive mode, whereby projection can be performed directly through the cell.
Devices employing liquid crystal materials have received considerable attention over the past several decades. The majority of these liquid crystal devices utilize liquid crystals of the nematic class. Such devices are incapable of data storage and must be continually addressed or refreshed. However, this is not a disadvantage where the desired display is dynamic and subject to substantially continuous updating.
A second class of liquid crystal materials, known as smectic liquid crystals, possess a storage capability in that image information need be written onto the liquid crystal cell only once. The written information is thereafter essentially permanent until erased.
Laser-addressed liquid crystal light modulators have been developed utilizing these smectic materials as high resolution projection display devices. The writing mechanism on these devices is primarily thermal. A focused infrared laser beam is used to heat the smectic liquid crystal material into the isotropic state. Afterwards, the liquid crystal is cooled back to smectic state and forms a light-scattering region. The written scattering region is stable within the smectic temperature range of the specific liquid crystal material, and the written information will be preserved. The cell can be erased by an electric field of the order of 105 volts cm or by a combined effect of heating and an applied electric field.
By projecting visible light through the cell following writing of the cell, any image written can then be projected onto a display screen, a photosensitive material, or the like. A variety of such display devices are discussed in Dewey, "Laser-Addressed Liquid Crystal Displays," Optical Engineering 23(3), 230-240 (May/June 1984).
Two types of laser-addressed liquid crystal light modulator devices have been developed for use in conjunction with an infrared diode laser. Both are discussed in the referenced publication of Dewey. One such device, known as a reflective device, utilizes a thin film infrared absorber fabricated on one of the substrates of the liquid crystal cell. As the infrared beam is scanned across the absorber layer, the radiation is converted to heat to produce scattering regions within the cell. However, the thin film infrared absorber is opaque, not only to infrared radiation but also to the visible radiation used in projecting the finished image. Thus, the image written on the liquid crystal cell must be projected by reflecting the visible radiation off the cell. This in turn necessitates the use of a relatively complex reflective optical system.
A second type of device is commonly known as a transmissive device. Typically, an infrared absorption dye having its peak absorption at the laser wavelength is doped into the liquid crystal material. As the writing beam is scanned across the cell, the dye absorbs the laser radiation and converts it to heat. The dye has little or no effect on light of a visible wavelength. Thus, the image written on such a device can be projected simply by passing projection light through the device in a manner similar to that used in an ordinary slide projector. The necessary projection system is therefore simpler and less expensive than in the case of reflective devices. However, the infrared absportion dye used in such device can be bleached after repetitive uses, and thus is inadequate for many applications.
What is needed, therefore, is a laser-addressed liquid crystal light modulator which overcomes the disadvantages of both the transmissive and reflective devices described above. Specifically, such a device should be able to use the relatively simple optical system of the transmissive device. However, the device should provide a long useful life without exhibiting reductions in performance.